A photovoltaic (PV) cell is generally an electronic device that is capable of converting incident light into electricity (direct current). A photovoltaic cell comprises a pair of electrodes and light-absorbing and charge transporting photovoltaic materials disposed therebetween. When the photovoltaic material is irradiated with light, electrons that have been confined to an atom in the photovoltaic material are released by light energy to move freely. Thus, free electrons and holes are generated. The free electrons and holes are efficiently separated and transported to the electrodes through charge transporting materials so that electric energy is continuously extracted. Current commercial photovoltaic cells use a semiconductor photovoltaic material, typically silicon. However, implementing silicon for photovoltaic cells generally requires high product cost due to extensive material and energy consumption. Another type of commercial PV cell that has lower material consumption are thin film PV cells such as a-Si, CaTe, CIGS, etc. However, these thin film PV cells generally require a high vacuum manufacturing process, which generally leads to high capital investment and operational expenses.
One alternative type of PV cell that has low cost potential is an organic or/and organic/inorganic hybrid cell. Among this class of PV cells, dye sensitized solar cells (DSSCs) may be the most promising for commercialization based on currently available experimental results. The DSSC has three major active materials: a dye, an electron transporter material (such as titanium dioxide) and a hole transporter material (such as electrolyte). The dye is generally used, because titanium dioxide (TiO2) alone absorbs little photon energy from sunlight. To sensitize the titanium dioxide, a dye (or chromophore) is coupled onto the surfaces of the semiconductor solid (e.g. titanium dioxide). When a dye molecule absorbs a photon, electrons are excited into the lowest unoccupied molecular orbital, from which they are injected into the conduction band of the semiconductor (e.g., titanium dioxide). Once in the conduction band, the electrons can then flow through a first electrode (also known as the front electrode, anode or photoelectrode). Thus, the semiconductor serves as a transport medium for electrons. Hole transport between the dye layer and the second electrode (also known as the back electrode, cathode or counter electrode) occurs through an electrolyte solution disposed between the electrodes. Practically, the returning electrons at the second electrode effect a oxidation-reduction (“redox”) reaction, generating a reduced species that returns the electrons to the oxidized dye molecules, and the cycle repeats. It is desirable to provide a sensitizing agent that absorb as large a portion of the sunlight wavelength as possible to maximize the harvest of photon energy.
A solar cell is a specific type of photovoltaic cell that is configured to convert solar energy (sunlight) into electricity. A solar cell includes two electrodes, which may be referred to as the solar electrode, configured to receive sunlight, and a counter electrode. The solar electrode collects high energy electrons from the photo-generation process. The counter electrode provides low energy electrons to the active cell materials disposed between the electrodes. Solar energy has become an attractive source of energy for remote locations and is widely recognized as a clean, renewable alternative form of energy.
As will be appreciated, solar energy includes a wide range of photon energies. To achieve high efficiency, the solar cell must absorb a sufficient amount of photons from the solar energy. There are two approaches to achieve maximum solar absorption. One is to use a dye that has very broad wavelength absorption. Another approach is to use several complimentary dyes. It is very difficult to have a dye that is both strong and a broad absorber. A strong absorber enables a thinner layer cell, thus provides a higher efficiency cell as a result of the short charge transport distance. Advantageously, by stacking multiple cell modules coupled in series, improved efficiency and appearance can be realized. Since each cell module in the stack may be configured to absorb a specific range of solar energy, it is possible to convert more photon energy to electric energy. Furthermore, the stacked configuration provides a more aesthetically pleasing device appearance.
Typically, in order for tandem cells, such as inorganic thin film cells, to achieve a high efficiency, great efforts are made so that each cell in series has the same or similar current. Otherwise, the device current will be limited to the smallest cell current. Light losses throughout the photovoltaic device, cost, material and processing limitations and interconnection among various elements in the photovoltaic device present a number of challenges in designing viable, useful, efficient, manufacturable and reliable photovoltaic devices. In contrast, organic or hybrid PV cells, can be fabricated at a comparatively low cost. Thus a new type tandem device can be made through a parallel tandem architecture where several layers of PV cells are stacked optically in series and electrically in parallel. One major advantage of this parallel tandem device architecture is that there is no requirement for current matching at each layer of the device. Furthermore, this architecture provides practical means for providing a variety of color appearances, because different layers of cells can be independently stacked. The flexibility of this architecture design requires unique ways for simple and low cost interconnects to match current or voltage for the integration of each of the layers of cells which make up the stacked device.